fix jlreq abstract layout and adapt Chinese docs for unicode engines

fix jlreq abstract layout and adapt Chinese docs for unicode engines

jlreq layout defined the abstract environment to be boxed
in the preamble. Doing any kind of typesetting before
all the setup has finished is error prone and may lead to
wrong output (wrong fonts for example). The correct
place to put the abstract code is right before \maketitle.

The Chinese docs loaded the ctex package in case system fonts
were used. Now, with the new support for languages with babel
and polyglossia it is no longer necessary and creates collisions.